Among the 860 exoplanets documented so far, a good portion of them are so-called super-Earths — planets that feature a mass greater than Earth's, but lower than our solar system's smaller gas giants, namely Uranus and Neptune (both of which are about 15 Earth masses). And as the name implies, some of these super-Earths have a high proportion of rock — a characteristic that makes them intriguing candidates in the search for extraterrestrial life.

Or not. As a new paper published in the Royal Astronomical Society suggests, these large terrestrial planets may have atmospheres so deep that we'd be better off calling them mini-Neptunes.

The new study, which was led by Helmut Lammer of the Space Research Institute (IWF) of the Austrian Academy of Sciences, suggests that low density super-Earths are clinging to an extended hydrogen-rich atmospheric layer. These planets, therefore, are unlikely to ever become Earth-like.

Lammer and his team reached this conclusion after analyzing the effects of radiation on the upper atmospheres of super-Earths orbiting the stars Kepler-11, Gliese 1214, and 55 Cancri. All these planets are in relatively close orbits with their parent stars, and they're all suspected of containing solid cores surrounded by an atmosphere rich in hydrogen, water, and methane. The astronomers theorize that these primordial gasses were captured from nebulae during planet formation but have not had a chance to escape — nor will they ever.

Their subsequent analysis showed that the short wavelength extreme ultraviolet light coming in from the planets' respective stars are heating up the atmospheres. As a result, the envelopes are bloating up to a massive size — over several times the radius of each planet. Some of these gasses have escaped into space (in a complex process called "hydrodynamic blow-off"), but most of the protoatmosphere remains intact.

"The atmospheric mass loss of the studied super-Earths is one to two orders of magnitude lower compared to that of hot Jupiters," they write," so one can expect that these exoplanets cannot lose their hydrogen envelopes during their remaining lifetimes."

This is potentially bad news as far as habitability is concerned. It's an open question as to how life-friendly planets like these can become — even if it's situated within a solar system's habitable zone. And worse, the further away the mini-Neptune is from the habitable zone, the larger its atmospheric layer.

The entire study can be seen at the Monthly Notices of the Royal Astronomical Society.